Superconductive magnets include those superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics. Known superconductive magnets include liquid-helium-cooled and cryocooler-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a first thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet preferably also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first cold stage in thermal contact with the thermal shield, and having its second cold stage in thermal contact with the superconductive main coil. A liquid-helium-cooled magnet preferably also includes a liquid-helium vessel surrounding the superconductive main coil and a second thermal shield which surrounds the first thermal shield which surrounds the liquid-helium vessel.
Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped superconductive coil assembly having a bore. The superconductive coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils are thus designed to create a magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet's bore where the object to be imaged is placed. A single, tubular-shaped superconductive shielding assembly may also be used to prevent the high magnetic field created by and surrounding the main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Such shielding assembly includes several radially-aligned and longitudinally spaced-apart superconductive shielding coils carrying electric currents of generally equal amperage, but in an opposite direction, to the electric current carried in the main coils and positioned radially outward of the main coils.
Open magnets, including "C" shape magnets, typically employ two spaced-apart superconductive coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. Known open superconductive magnet designs having shielding include those wherein each superconductive coil assembly has an open bore and contains a superconductive shielding coil positioned longitudinally and radially outward from the superconductive main coil(s). A large amount of expensive superconductor is needed in the main coil to overcome the magnetic field subtracting effects of the shielding coil. Calculations show that for a 0.75 Tesla magnet, generally 2,300 pounds of superconductor are needed yielding an expensive magnet weighing generally 12,000 pounds. The modest weight makes this a viable magnet design.
It is also known in open magnet designs to place an iron pole piece in the bore of a superconductive coil assembly which lacks a superconductive shielding coil. The iron pole piece enhances the strength of the magnetic field and, by shaping the surface of the pole piece, magnetically shims the magnet improving the homogeneity of the magnetic field. An iron return path is used to connect the two iron pole pieces. It is noted that the iron pole piece also acts to shield the magnet. However, a large amount of iron is needed in the iron pole piece to achieve shielding in strong magnets. Calculations show that for a 0.75 Tesla magnet, only generally 200 pounds of superconductor are needed yielding a magnet weighing over 70,000 pounds which is too heavy to be used in medical facilities such as hospitals. The weight does not make this a viable magnet design.
What is needed is an open superconductive magnet design having shielding which is light enough to be used in medical facilities and which is less expensive than known designs.